Production of ethanol by microorganisms provides an alternative energy source to fossil fuels and is therefore an important area of current research. It is desirable that microorganisms producing ethanol, as well as other useful products, be capable of using xylose as a carbon source since xylose is the major pentose in hydrolyzed lignocellulosic materials, and therefore can provide an abundantly available, low cost carbon substrate. Zymomonas mobilis and other bacterial ethanologens which do not naturally utilize xylose may be genetically engineered for xylose utilization by introduction of genes encoding 1) xylose isomerase, which catalyses the conversion of xylose to xylulose; 2) xylulokinase, which phosphorylates xylulose to form xylulose 5-phosphate; 3) transketolase; and 4) transaldolase.
There has been success in engineering Z. mobilis cells for xylose metabolism (U.S. Pat. Nos. 5,514,583 , 5,712,133, 6,566,107, WO 95/28476, Feldmann et al. (1992) Appl Microbiol Biotechnol 38: 354-361, Zhang et al. (1995) Science 267:240-243), as well as a Zymobacter palmae strain (Yanase et al. (2007) Appl. Environ. Mirobiol. 73:2592-2599). However, typically the engineered strains do not grow and produce ethanol as well on xylose as on glucose. Strains engineered for xylose utilization have been adapted by serial passage on xylose medium, resulting in strains with improved xylose utilization as described in United States Patent Application 20030162271 and commonly owned and co-pending US Patent Application Publication No. US 2008-0286870 A1. It has been shown that these improvements were the result of selection for altered sequences for improved expression of the pGAP promoter regulating expression of the xylose isomerase gene. Those sequences and methods for their use in improved expression of transgenes in Z. mobilis are disclosed in commonly owned and co-pending United States Patent Application Publication Nos. US2009-0246876 A1 and US2009-0246846 A1.
It is desired to use cellulosic hydrolysates as a renewable source of sugars for fermentation media for production of ethanol by biocatalysts. Cellulosic hydrolysates, which are generally produced from biomass by pretreatment and saccharification, typically contain substances that are detrimental to biocatalyst growth and production. For example, acetate is a common product present in cellulosic hydrolysates which has been shown to be inhibitory to Z. mobilis at concentrations routinely found in hydrolysate (Ranatunga et al. (1997) Applied Biochemistry and Biotechnology 67:185-198).
There remains a need for strains of Zymomonas, and other bacterial ethanologens, which have maximized xylose utilization in the presence of stresses imposed by impure sugar sources produced through saccharification of biomass.